Japanese legislature had approved a bill that would be a big blow to the Piracy in the country, with penalties on the downloading of the copyrighted material. Once a Law it could end up some lads in jail for two years and fine of $25,000. Now that was enough to make the Anonymous angry.... The Robin Hood of internet freedom and .. you can say now piracy freedom too, Hacked many Japanease Govt Site!

Anonymous took the full responsibility of the attack over their Twitter channed @op_japan . They hit many sites including finance ministry , Supreme court and some political Parties Sites.
They said following in the message :

"To the government of Japan and the Recording Industry Association of Japan, you can now expect us the same way we have come to expect you in violating our basic rights to privacy and to an open internet.

According to the Recording Industry Association of Japan, 4.36 billion files were illegally downloaded in the country in 2010. So all the guys who were worried they should know some one is there for them , when rain is falling down (Read it > Bill is falling down!)